A purple tapir and Kuromi's sidekick with the ability to fly. He, in some episodes, appears to have a crush on Kuromi, and is often victim to Kuromi's anger. Kuromi is frequently very physically abusive to him, but regardless, she truly does love him, although despite the fact that they're both animals, they have a master-pet relationship.

Like Kuromi, Baku has a journal where he writes down his moments with her. Hiiragi burns it in the middle of the first season, but he somehow retrieves it in the second season when they break out of jail.

His birthday is confirmed to be on February 29. His name is a bit of a pun: in Japanese, baku could mean a tapir or a dream-eating creature of Japanese mythology. In one episode he has a crush on Uta. He does appear to have a conscience. On one occasion, he was seen delivering newspapers and giving rides to children. Baku has the ability to sniff out Black Notes, and whenever a new one is obtained, Kuromi skewers it with a magical stick and Baku eats it for safekeeping. Baku first met Kuromi at a beach in Mari Land. Baku usually says "Zona" at the end of his sentences, a lot like a catchphrase which is common of anime characters.